The present invention relates generally to the lost pattern process of investment casting, and more specifically to the formation of the pattern assemblies used in such a process.
In the lost pattern process, a pattern assembly is conventionally prepared by attaching a plurality of patterns around a sprue member or center tree to form a branched "set-up". A pouring cup or well is frequently attached to the center tree. The patterns, which are replicas of the parts to be cast in metal and include necessary gates and risers, are made of an expendable material, such as wax, a synthetic resin, or a wax and synthetic resin composition. The pattern set-up is coated or invested with a suitable slurry which is allowed to harden to form a refractory mold. Thereafter, the patterns are destroyed, such as by subjecting the mold to heat or a solvent for the pattern material or both to form the mold cavities.
An improved sprue constuction for use in investment casting processes is disclosed in U.S. Pat. No. 3,520,349 and in U.S. Pat. No. 3,520,350 issued July 14, 1970 to C. H. Watts. A preferred form of the improved sprue construction disclosed in those patents comprises a tube made of cardboard or plastic and a corrugated sleeve around the tube. The corrugated cardboard sleeve has corrugations on its inner side in contact with the tube and a smooth outer surface. In order to locate the positions of the patterns to be attached to the sprue member, a sheet of perforated paper on which suitable pattern locating markings are stamped or otherwise printed may be wrapped around the outer surface of the corrugated cardboard sleeve. A thin coating of wax is applied to the perforated paper in order to secure it to the underlying corrugated cardboard sleeve and also to provide a wax surface to which the patterns can be conveniently attached. In the case of small diameter sprue constructions, such as are used when the sprue cavity in the mold is to be cast solid with metal, the corrugated cardboard sleeve may be replaced by a sheet of heavy paper having a very thin wax coating on its outer surface, the perforated paper having the pattern locating markings being applied over the heavy paper as previously described.
Although both of the sprue constructions described above work well in most instances, a problem can arise when they are used for mounting large, heavy patterns. Heavy patterns may cause some flexing of the sleeve whether it is made of heavy wax-coated paper or corrugated cardboard, and this flexing may permit the patterns to come loose from the sprue member so that refractory slurry penetrates beneath them and becomes a source of dirt in the metal castings. The patterns may even become so loose as to fall off of the set-up during processing.
As a practical matter, it is not desirable to attempt to correct the flexing problem by applying a thick layer of wax over the flexible cardboard or paper sleeve. The inner diameter of the mold passage formed by the sprue member is determined by the thickness of the wax layer, and the thickness is more easily controlled by applying a very thin coating. The dimension of the sprue passage in the mold is important because it usually must mate with a core or a plug at one end to prevent metal leakage. When a core is disposed in the sprue passage of the mold as disclosed in U.S. Pat. No. Re. 26,495, the critical spacing between the core and the inside of the mold depends upon the thickness of the wax layer on the sprue and needs to be accurately controlled. Another reason why it is undesirable to use a thick wax coating on the sprue construction is that it is desired to peel the flexible sleeve and the perforated paper from the inside of the mold to promote the pattern removal operation. It is also desired to reduce contamination of the melted pattern material with paper or cardboard in order that the pattern material can be reclaimed conveniently.